A data center is a facility that houses servers and other computing equipment for large-scale applications. The expectation of uptime for equipment in a data center is very high. Moreover, the equipment, and the software that runs on the equipment, generally is not designed to shut down gracefully in the case of an abrupt power failure. Thus, data centers are generally equipped with backup power systems to ensure continuous delivery of power.
One type of component that is used to implement a backup power system is an Uninterruptable Power Supply (UPS). A UPS uses a flywheel or battery to store sufficient energy to compensate, temporarily, for disruptions in the power supply. For example, if utility power is lost, the data center may have an on-site backup generator, but may take a few seconds to start the generator. Thus, the UPS keeps the equipment running while power is switched from utility power to on-site generator power.
The UPS's capacity to deliver power acts as kind of a bottleneck for the data center. Even though the amount of time that the equipment is running on UPS power is short (normally measured in seconds), the UPS has to be able to deliver enough power to meet the demands of the equipment. Since power failures are normally unexpected, it is unknown exactly when the UPS will be called on to provide power, so the UPS has to be ready to deliver enough power when that power is called for. Although over-sizing of UPS capacity can help to ensure that the UPS can provide enough power, UPS equipment can be expensive, so significant over-sizing might not be feasible.